Wireless networks, such as wireless local area networks (WLAN) typically communicate via radio or other frequencies. In a WLAN, for example, mobile stations are typically moving around, and may communicate with an access point (AP) or base station. The AP is typically a fixed device that may (or may not) be connected to infrastructure networks or wired networks.
The most common WLAN technology is described in the Institute of Electrical and Electronics Engineers IEEE 802.11 family of industry specifications, such as specifications for IEEE 802.11b, IEEE 802.11g and IEEE 802.11a. A group of 802.11 mobile stations may be in communication with each other (either directly or through one AP) in a network known as a basic service set (BSS), which may be identified by a basic service set identifier (BSSID). A group of BSSs (e.g., with one AP per BSS) may be coupled together in a larger WLAN network (e.g., with multiple APs) known as an extended service set (ESS), which may be identified by a service set ID (SSID).
When a mobile station moves around a wireless network (e.g., WLAN), the station may need to change its present association from one AP to another AP if the signal reception level of the presently associated (serving) AP becomes too low and/or another AP provides a better signal level or capabilities. This procedure is known as roaming, and allows a mobile station to switch association among APs.
Before roaming or associating with a new AP, a mobile station may first locate one or more APs. There are a number of known techniques for measuring or monitoring radio signals to allow roaming between APs. Signals transmitted by APs may generally be monitored, and the received signal strength (RSS) may be measured for each AP, for example. In particular, APs may be located, for example, via passive scanning and by active scanning.
In passive scanning, a wireless station may monitor one or more channels to determine if there is a beacon or probe response transmitted by APs. A beacon may be transmitted by each AP at regular intervals (known as beacon intervals), e.g., every 100 ms. A beacon (or beacon signal) may include, for example, an AP timestamp representing the value of the timing synchronization function (TSF) timer to keep the associated stations synchronized to the AP, a BSSID identifying a specific WLAN network, a traffic indication map (TIM), beacon interval (or amount of time between beacon transmissions), an indication of the supported data rates, parameter sets or information about specific signaling methods that are supported, capability information (such as whether Wired Equivalency Privacy or WEP protocol is supported), and other information. In passive scanning, a node may listen for beacons or probe responses on each of a plurality of channels, gathering information on each AP such as received signal strength, capabilities and data rates of the AP, etc. In some instances, passive scanning may expend time and power while listening for a beacon or probe response that may never occur or may be an idle channel.
Active scanning may involve a mobile station transmitting a probe request to one or more specific APs, and receiving probe responses from the APs. Active scanning allows mobile stations to interact with APs or other stations to obtain information. A mobile station may transmit, for example, a probe request that specifies a service set identity (SSID) in the probe request frame. Any AP or station on the channel that matches the SSID may, for example, respond with a probe response. The probe response, similar to a beacon, may provide information that may be useful to allow a station to access the network or associate with an AP. When compared to monitoring of beacons (passive scanning), active scanning may, in some cases, result in longer battery life (e.g., may consume less battery power), but also may reduce network capacity due to the probe request/probe response traffic.
It has also been recently proposed for APs to transmit measurement pilots (which sometimes may be referred to as gratuitous probe responses). An AP may typically transmit a measurement pilot frame much more frequently (e.g., every 10 ms) than a beacon (e.g., every 100 ms) to provide more frequent information to stations. Thus, because the measurement pilot frame interval is typically much shorter than a beacon interval, stations may use measurement pilot frames to detect whether there is an AP transmitting on the channel, to measure the channel quality (e.g., measure received signal strength), and to receive other information relating to the AP in a much shorter period of time, e.g., as compared to waiting for the next beacon on the channel. Thus, at least in some cases, measurement pilot frames may facilitate quicker roaming decisions and association to a new AP. The measurement pilot frame may contain much of the same information provided in a beacon or probe response, but the measurement pilot is shorter since it may not include some of the longer fields, such as all of the TIM and DTIM fields provided in the beacon. Measurement pilot frames may be received by a mobile station, for example, during active or passive scanning.